A gel by definition is a sponge like, three-dimensional solid network whose pores are filled with another substance, such as a liquid. The liquid of the gel is not able to diffuse freely from the gel structure and remains in the pores of the gel. But when the gel is subjected to a drying process, the liquid may be removed from the network, thereby leaving the solid network behind.
Drying the gel using conventional drying techniques results in formation of a xerogel. A xerogel is a solid formed from drying a gel with unhindered shrinkage. The associated shrinkage of the solid network associated with conventional drying techniques is caused by capillary forces acting on the pore walls as the liquid evaporates, and such shrinkage generally results in the destruction of the initial solid network. Xerogels are generally characterized as having a porosity of about 30% and in some embodiments, may have a specific surface area of about 500 m2/g.
By contrast, drying the gel using a supercritical drying or freeze drying process can yield an aerogel. An aerogel is a porous solid that is formed from a gel, in which the liquid that fills the pores of the solid has been replaced with a gas. Aerogels are generally produced by drying the gel either by a supercritical drying or by freeze drying. Shrinkage of the gel's solid network during drying is negligible or all-together prevented due much in part to the minimization of capillary forces acting on the network as the liquid is expended.
Aerogels are generally characterized as having high porosity (about 94-98%), and high specific surface area. Aerogels also possess relatively low densities, generally in the range of 0.004-0.5 g/cm3. Aerogels can possess excellent load bearing properties and insulation properties, and may be used as a catalyst or in connection with a catalytic process (e.g., as a catalyst support structure).